The end of the Cold War and the ascendance of transnational terrorism have fundamentally changed the nature of the threat posed by nuclear weapons to the United States (and to other nations). What remains constant is the understanding that the detonation of a single nuclear device on American soil would have immediate and profound impacts, the scale and breadth of which are difficult to comprehend. Given the openly professed desire of terrorist organizations to obtain nuclear weapons and their demonstrated willingness to sow widespread destruction, serious efforts are clearly required to assess this nation's vulnerabilities and to enhance our security posture. One potential class of vulnerabilities are U.S. seaports where currently an average of 15,600 cargo containers arrive by ship every day, any one of which could be used by an adversary to conceal fissile material or an assembled nuclear device.
The currently prevailing model for addressing threats associated with cargo-container-borne fissile material could be characterized as a customs-based approach, where radiation detection systems are integrated into the existing customs infrastructure at ports. The system scans suspect containers as they are unloaded from the ship or as they are subsequently loaded onto a truck or train for inland transport. During this scanning operation, the system utilizes either active or passive techniques to detect nuclear signatures from fissile material in the containers.
The passive scanning technique utilizes detectors to scan for the normal radiation signal from fissile material, while the active scanning technique sends a beam of neutrons or photons towards the cargo causing a unique return signal from subsequent fission events. Most systems have focused on the placement of detector arrays in a wall on or near the docks. Some of the ground-mounted arrays are situated such that semi-trucks or trains pass through detector portals along their normal routes. Almost all of these systems require the vehicle to stop for a few minutes so that a statistically significant number of counts can be collected.
The customs-based approach is a natural extension of conventional strategies for finding and seizing incoming contraband. Nuclear weapons, however, are unlike any other kind of contraband in that their power is so great, and their effects so far-reaching, that discovering them in port (or even allowing them to enter port at all) cannot provide adequate protection from their destructive reach.